Smoothing servo



Sept. 7, 1948."

Filed A ril 12. 1946 F- P- COFFIN, JR, ETAL SIOOTHING SERVO 's'sheets-sneet 1 TARGET' -'-CENTER OF AREA -CENTER OF GATE p-OC CENTER OF AREA TRUE TARGET POSITION CENTER OF GATE g INVENTORS FRANCIS F! COFFINIR PRESCOTT O. CROUT FRANK E. BOTHWELL Sept. 7, 1948. P; COFFIN, JR, ETAL I 2,449,035

suoownme smwov Filed Apr11'12. 194s I. 3 Sheets-Sheet 2 1 F'IG.3

Y INVENTORS mucus P. coFFlNJn PRESCOTT o. GROUT FRANK E. BOTHWELL ATTQBNEY' Sept. 7, 1948. F. P. COFFIN, JR, 2,449,035

' suoomme sgnvo Filed April 12. 1946 .3 Sheets-Sheet 3 a 52 5: i A :3 R4 0 s5 FRANCIS P. coFFmn PRESCOTT acnour FRANK EBOTHWELL BYIW ATTORHEY l i INVENTORS Patented Sept. 7, 1948 SMOOTHING SERVO Francis P. Coflin, Jr., Schenectady, N. Y., and ,Frank E. Bothwell, Boston, and Prescott D.

Crout, Lexington, Mass, a'ssignors, by mesne' assignments, to the United States of America as represented by the Secretary of the Navy Application April 12, 1946, Serial No. 661,594

4 Claims. (Cl. 31818) This invention relates in general to apparatus for the intepretation of electrical data, and more specifically to a novel circuit for smoothing the instantaneous erratic fluctuations of a servo mechanism input error signal, which fluctuations tend to obscure the normal, desired signal variaons.

Data smoothing circuits of the type herein described find application in electrical equipment whenever the normal signal trend is disturbed by discontinuous jitter." For the purpose of claritying the discussion of this circuit, reference is made to a particular radar problem, which is representative of'data smoothing problems encountered in practice. Thus consider a radar installation utilizing a spinning, directional antenna for the detection of targets. A plurality of target echoes are obtained as the radiated, pulsed beam sweeps a single reflecting object such as a ship. Factors influencing the number of echo pulses are the radar pulse repetition frequency, the directivity of the antenna beam, the azimuth angle subtended, by the target ship and the angular speed of the antenna. It may be expected that the relative intensity of a single echo pulse would be solely a function of the radiated beam inten-' sity at the particular instant considered, that the video envelope of all echoes obtained from a single target during a scan would comprise a smooth wave, having a maximum at a point corresponding to the point of maximum radiated energy in the directional beam, and that the motion, in time, of this envelope would conform to the actual angular movement of the ship. Such considerations have been found contrary to fact; and it has been observed that echoes obtained from a fixed target ship dolnot lie under a smooth envelope,

and that the apparent echo maximum, and the envelope center of area are highly erratic functions, and "jitter" from scan to scan. A theoretical analysis indicates that this eifect should be expected due to the variable interference patterns obtained as the reflecting ship changes its relative position and aspect due to rolling, pitching, and the like. Thus onone scan the video envelope of all reflection might have its maximum at the true azimuth of the ship. On the following scan the video envelope might erroneously indicate a shift in azimuth in one direction, and the next 'scan a further shift by a different amount.

It is apparent that such a jitter in the azimuth electrical data would preclude the use thereof for an automatic azimuth tracking system. It is therefore an object of this invention to provide a data smoothing circuit and servo system capable sign of which is governed by the synthesis of a oi delivering an output indicative of the input signal trend and substantially independent of the Jitter thereof. This desired result is achieved by an electro-mechanical system wherein input and output are interrelated by. a predetermined equatlon of motion, involving inertia and damping.

Accordingly, a further object of the present invention is to provide an electrical circuit, the dedifferential equation of motion which satisfies the predetermined data smoothing requirements.

Another object of our invention is to provide apparatus for smoothing radar data obtained from a reflecting target during uniform antenna scans, such that automatic target tracking upon the smoothed signal is possible.

These and other objects of our invention will now become apparent from the following specification taken in connection with the accompanying drawing in which:

Fig. 1 is a graphical illustration of the idealized radar response plotted as a function of scanning angle;

Fig. 2 is a graphical illustration of a radar respouse and shows the condition normally obtained in practice; j

Fig. 3 is. a detailed schematic circuit diagram of the novel data smoothing servo mechanism of the present invention; and

Figs. 4, 5, 6, 7 and 8 are circuit breakdowns of the data smoothing circuit illustrated in Fig. 3 for the purpose of properly explaining the operation thereof.

Referring now to Fig. 1, there is graphically illustrated the idealized radar response as obtained by a fixed antenna scanning a fixed target.

ceived pulses is, for this idealized example a smooth curve, symmetrical about a maximum pulse echo. extent of the angle covered by the envelope i6 is a function of the various factors such as radar beam width and target angle as hereinabove discussed. In this example, the angular position I! of the maximum received pulse is representative of the target bearing and corresponds with the axis of symmetry passing through the center of area under the pulse-envelope Hi. There is also illustrated a rectangular gate 2| symmetrically disposed with respect to the pulse envelope l6 and representative of an angle duringwhich period the target in question is illuminated and during The amplitude of a pulse and the rectangular voltage block, the duration of which corresponds to the time required for the antenna to scan the angle covered by the gate. The voltage gate switches on the indicating circuits.

Actually the ideal scanning situation illustrated is seldom obtained. A truer picture thereof is illustrated in Fig. 2 wherein the envelope 22 of the received echo pulses 23 is a highly irregular function of the scanning angle for reasons having as a basis the interference of the reflected waves. The axis 24 indicating the angular position of the center of area under envelope 22 is generally not midway between the angular extremes of the envelop 22, and neither the center of area 24 nor the peak echo corre-' sponds with the true Position 25 in azimuth of the target. A rectangular gate 26 for illuminating the particular target in question is illustrated having its position symmetrical relative to the true target position 25. The problem encountered is thus, the maintenance of the center v of gate 26 in coincidence with the true target position 25 at all times and independent of the apparent jitter of the response center of area 24. For this purpose it is necessary to smooth the recurrent data, such as that shown in Fig. 2,

-obtained upon successive scans of the antenna.

In Fig. 3 there is shown a preferred embodiment of a, data smoothing circuit and servo system capable of providing a mechanical output which corresponds to the general trend of signal variation applied thereto. The operation of this servo, in accordance with the principles of the present invention, is governed by an equation of motion, that is, a relation between the electrical information applied at input terminal 3| and the mechanical output at rotatable shaft 32, as follows:

(1) Ijj+c, ,=zz. o +nf wherein, and 0 are representative of the input and output, respectively, of the servo, and wherein I, C, K1, and K2 are constants. A study of this equation indicates that the first term on the right hand side involves the difference between input and output, that is, 0, and is accordingly proportional to the system error, a factor to be discussed in greater detail hereinbelow. The first and second terms of the left hand side include the second and first derivatives of the servo output, .and are from elementary mechanical considerations characteristic of inertia and damping respectively. It has been observed that servo having an equation of motion involving solely the three terms hereinabove discussed, has the undesirable feature of a lag error. This is compensated by the addition of the second term on the right hand side of Equation 1, proportional to the integral of the error signal.

The servo system illustrated in Fig. 3 comprises essentially the results of the synthesis of Equation 1. When utilized in connecti n with .response is given by the left hand member.

the special radar azimuth tracking system hereinabove mentioned, the mechanical output at shaft 32 is utilized to position the tracking gate 26, Fig. 2, and substantially maintain the azimuth thereof on the true target position.

The input signal applied to the servo at terminal 3| is derived from the radar target data received, having the waveform illustrated by the succession of pulses 23 under envelope 22 Fig. 2. This input signal is in the form of pulses, at the repetition rate of the pulses 23 and is of a nature such that each pulse appearing at terminal 3| is instantaneously proportional, insofar as amplitude is concerned, to the product of the displacement of the corresponding pulse 23 from the center of gate position 25 and the amplitude of the same pulse 23 of the envelope 22. This input signal is graphically illustrated by the waveform 33 at terminal 3| Fig. 3. It is to be noted that the pulses 33 are plotted as a function of time, whereas the pulses 22 are shown as a function of the scanning angle, a. However, for an antenna spinning at a constant speed, there is a direct, linear relationship between motion in azimuth and time The particular system for obtaining the input signal 33 utilizes, in one embodiment thereof, two sawtooth voltages, one rising and one falling during the period of time represented by the gate 26, which voltages are combined with the radar echo pulses. However, the means for generating this waveform, forms no part of the present invention and will not be discussed further. A series of pulses, such as 33, appears at terminal 3| during each scan of the antenna. For an antenna rotating at normal scanning speed the time period of "illumination of a target, and accordingly the period of application of pulses at terminal 3| is sniall. The rate of application of signals to terminal 3| corresponds to the antenna scanning 'rate. The total energy content of the pulses 33 obtained during each scan is directly proportional to the error, or deviation of the center of area 24 from the center 25 of the illuminating" gate 26 of a pulse group such as shown in Fig. 2, and therefore this signal is herein called the error signal.

A cathode follower, comprising triode 35, having the plate thereof connected to a positive, regulated power supply line 36 and employing cathode load resistor 31 is used as a high impedance input circuit to the servo system This is followed by a stage of amplification consisting of triode 4|, the plate of which is connected to the positive source 36 through load resistor 42. The pulsed signal is coupled to the amplifier 4| by connecting its cathode to that of cathode follower 35. A positive bias is placed upon the grid of amplifier 4| by voltage divider resistors 43 and 44, which bias provides a normal negative grid relative to cathode for the coupling system employed.

The amplified pulsed error signal appearing at the plate of amplifier 4| is coupled to the smooth-- ing circuit, which functions in accordance with the differential equation of motion (1). The steps involved in the synthesis of a physical system having this equation of motion are now preesented to facilitate the understanding of the circuit of Fig. 3.

In Equation 1 consider the right hand member thereof as a voltage applied to a network whose In Fig. 4 there is illustrated a resistance capacitance network having an input applied voltage E1 and and a plying Kirchofi's laws, the following system of equations is established:

a a+ c i E, er.

Eliminating Ev, i1, and is in Equations 2 gives dE, (RIRZCICQW Thus, the network of Fig. 4 provides an equation of motion of the desired form with the exception that the driving term is EiEo, Equation 3, in-. stead of E1.

In Fig. 5, there is illustrated a modification of the circuit shown in Fig. 4 which functions to correct for this difierence in the driving term. In this network, the capacitor C2 is disconnected from its common connection with capacitor Cl and connected instead to the movable tap ofa potentiometer Ra, which in turn is energized by a power source of polarity as indicated. The output voltage E is used to drive a position servo, in a manner to be described, which servo mechanically determines the movement and position of the potentiometer arm of Re. If the servo is operative suchthat the output voltage E0 is maintained at zero at all times, then, the voltage E across capacitor C: may be expressed by the differential equation:

Thus, for an applied driving potential E1, the circuit of Fig. 5 provides a voltage E, influenced by inertia and damping as indicated by the first and'second terms respectively of the left hand member of Equation 4. It is to be noted that if the potentiometer R3 is linear with respect to its shaft rotation, then the angular movement of the shaft also satisfies Equation 4. The application of this fact will be explained later.

For the synthesis of the right hand member of Equation 1, consider first the circuit illustrated in Fig. 6. If e; and co are the input and output voltages respectively, and using the symbols for the circuit constants as shown in the drawing,- then:

- e,-=e,-HR

The elimination of i-between these equations gives:

(6 ea fit-ma bulb ll, and resistor R4. the latter being the equivalent of R in Fig. 6. A capacitor C3 is connected to the grid of cathode follower 5|, and is equivalent to capacitor C in Fig. 6. The plate ofthe' cathode follower is connected to a positive power source and the cathode thereof 'is returned to the negative side of the power supply through a load resistor 68. To complete the circuit a resistor 64 is connected between the cathode of the cathode follower 5i and the junction .between capacitor 50 and neon bulb 52.

The circuit of Fig. 'l is dependent for operation upon the nature of the applied signal e. This signal constitutes the pulsed, intermittent error signal previously described in connection with Fig. 3. Thus in the period between input pulses, e1 is zero, and the capacitor 50 charges to the potential e0 through resistor 54. which is also equal to the potential across Ca. When an input pulse is applied such that 81O, the potential at the junction betweencapacitor 50 and neon bulb 62 instantaneously rises to the value ei+eo, causing the neon bulb 52 to break down applying this potential to the junction 55. This last statement assumes that the drop across the neon bulb 52 is negligible During the period of neon bulb breakdown a voltage e1 exists across R4. provided that capacitor 50 is sufllciently large so that its discharge during the pulse is insignificant and provided that there is little change in during the pulse The neon bulb 52 serves only as a switch separating capacitor Sll from capacitor C3 during the period when capacitor 50 is recharged through resistor Bl.

- The potential across C: is applied to the cathode follower 5|,and thus a substantially equal potential appears at the cathode, from which 80 is taken. As a result of the switching action of the bulb B2, thepotential across C3, and hence Co, is given by the equation:

1 t an; of indicating that the circuit electrically performs -the integration vof the error signal, as rethe voltage eo is given by: a

overall performance required fdr data smoothing.

In Fig. 8 the circuit components and potentials are designated as in Figs. 5 and '7. Thus, e1 is the input error signal operative to generate an output voltage E, which is properly smoothed.

The potential En is maintained identically zero by a servo (not shown) mechanically connected to the movable element of Ra.

The equation of motion of the system is given by:

which is of the form of Equation 1. The smoothing achieved by this circuit is a, function of the nature of the applied error signal c1, and the various circuit time constants. The time constants are determined by'the particular appli- 'cation from circuit design principles which will not be discussed.

The complete data smoothing circuit'diagram Fig. 3 incorporates the elements of the basic smoothing circuit developed in Fig. 8, and accordingly similar elements in these two drawings have been so designated. Thus the amplified error signal appearing at the plate of triode 4| is coupled through a capacitor 58 to the Junction of resistor 54 and a switching circuit 52 .which functions as neon bulb 52 inFig. 8. In practical operation, a neon bulb has the disadvantage of a comparatively large breakdown voltage. In Fig. 3 the neon bulb has been replaced by a double diode, biased rectifier circuit comprising the oppositely phased diodes II and 12. Thus the plate of diode 1| and the cathode of diode I2 are connected together and to the junction between capacitor 58 and resistor 54. The other cathode and other plate are connected through a filter capacitor 18 shunted by the equal series resistors 14 and I5. The junction point between resistors I4 and I5 is directly connected to the control grid of triode cathode follower 5|. The cathode of diode II is maintained at a positive potential relative to the plate of diode I2 by the output of a rectifier I6 which in turn is energized through transformer 11 the primary of which is connected across-a power line 8|. Diode I6 is a conventional half-wave rectifier having its cathode connected to one side of the secondary winding of transformer I1 'and its plate connected to the opposite side thereof through the resistors I4 and I5. Capacitor I3 serves to smooth the voltage developed across resistors I4 and I5 and of the polarity as indicated. A high frequency by-pass capacitor 82 is shunted between one side of the secondary winding of transformer 11 and ground.

The rectified voltage developed by diode I8 thus places the cathode of rectifier II positive relative to the junction between resistors 14 and I5 and the plate of diode I2, negative relative thereto by an equal amount. Thus neither diode 'II or I2 will conduct until the potential applied at the junction between capacitor 58 and resistor 54 exceeds the magnitude of the bias. Effectively, therefore, in Fig. 3 between terminals 83 and 84 the circuit functions as the neon bulb 52 of Fig. 8 with the exception that the switching potential is set at a lower, predetermined value.

The control grid of the cathode follower 5| is connected to ground through the series combination of R4 and C3. In accordance with the circuit principles disclosed in connection with Fig. 8, the potential at the cathode follower grid is equal to the error signal plus the integral of the error signal. The cathode of triode 5| is returned to ground through a total resistance 53 comprising the series combination of a comparatively small resistor 86 and a larger resistor 81. Resistor 54 is connected from the capacitor 58 to the junction between resistors 86 and 81. It will be noted that in Fic. 8 the resistor 54 was connected directly to the cathode of cathode follower 5|. In

-Fig. 3 the relation between resistors 86 and 81 .grid potential. This connection will preclude conduction in diodes II or 12 in the period between pulses.

In order to. provide a cathode follower plate voltage which is regulated and greater than that of source 38, a small neon bulb 8|. or other suitable discharge tube, is connected between the plate of triode 5| and voltage source 35. The junction between the neon bulb and the plate is connected to a high potential, unregulated power supply at terminal 82 through a dropping resistor 93. It is evident that the potential at the plate of triode 5| is equal to the regulated power supply voltage 38 plus the constant drop across neon bulb 9|. V

The voltage appearing across the cathode resistor 81 is coupled into a networkdeveloped in connection with Fig. 5, and includes series resistor R1 and shunt capacitor 01, series resistor R2 and shunt capacitor C2. As described in connection with Fig. 5, capacitor C2 is connected at one end thereof to the movable arm 'of a linear potentiometer Ra. The proper operating potential is obtained for R; by connecting it between the positive power supply 38 and ground through resistors 85 and 86 respectively. These last-mentioned resistors are preferably of equal magnitude for reasons which will hereinafter be disclosed. As is indicated by the broken line 81, the movable tap on the potentiometer R; is mechanically ganged to the rotor 32 of a servo motor IOI.

The position servo mechanism illustrated in Fig. 3 is a conventional unit and functions to maintain the potential at terminal I02 of capacitor C: relative to another fixed potential, identically equal to zero. The reference potential is obtained by a voltage divider network comprising equal resistors I05 and Ill-B in series with the parallel combination of potentiometers I81 and I88. A differential amplifier comprising a double triode circuit III and H2 is used to compare the potential at terminal I 82 connected to the grid of triode III and that of the variable tap N3 of potentiometer I81 connected to the grid of triode H2. The cathodes of the triodes I I I and I I2 are connected together and to ground through a common resistor Ill. The plates are returned to the positive regulated power supply through equal load resistors H5 and H6.

The servo unit IOI comprises essentially a twophase motor having one winding I2I connected to the input power supply 8| through transform er I22. The second motor winding I23 is shunted by a phase changing capacitor I24 and is shunted across one winding I25 of a saturable core transformer I26. The primary winding I21 of the transformer I26 is connected across the power supply 8|. A third winding I3I of the transformer I26 is connected between the plates of a double triode amplifier I32 and I33. The latter amplifier is energized from the power line 8| through transformer I35. As illustrated, the secondary of transformer I35 is connected between the center tap of the windin I3| and the comm-on cathode connection of triodes I32 and I33. These cathodes are biased at a positive value by a tap on pctentiometer I which is connected across the positive supply line 3-6 in series with resistor I42. The grids of triodes I32 and I33 are connected to the plates of the differential amplifier triodes H2 and III, respectively.

If the potential at terminal I02 at capacitor C2 is equal to'that at II3 then the triodes III and H2 conduct equally, and thus the potentials'of the grids of triodes I32 and I33 are equal. This in turn results in equal alternating plate currents in triodes I32 and I33, and as a result of the winding connection of transformer I26 does not energize the winding I23 of the motor .IOI. Accordingly, there is no rotation of motor shaft 32. If the potential at terminal I02 is disturbed and does not equal the potential at terminal II3, the differential amplifier III, II2 will apply grid potentials to the triodes I32, I33, which will result in an output coupled through transformer I26 that causes motor rotation in that ndilECtiOl'l which drives the movable arm of potentiometer Ra such that the potential at terminal I02 is made equal to that at terminal I I3. If the movable tap at terminal II3 on potentiometer is adjusted so that the potential thereat is equal to one-half of the supply potential 36 then the normal zero position of the potentiometer R: will be at the midpoint thereof since resistors 95 and 96 are equal.

It is evident that the potentials of capacitor C2 and capacitor, C1 must be brought to the potential of terminal II3 before operation of the system may be started. For this purpose, switch I5I is closed, thus energizing relays I52 and I53, and closin contacts I54 and I55. Closure of these contacts completes a connection from the variable tap of potentiometer I08 to the capacitors C1, C2 and C3. The potentiometer I08 is then adjusted so that the capacitor potentials are equal to the potential at the terminal II3 following which switch I52 is opened. It is evident that for proper operation the insulation of capacitors C1, C2 and C3 must be extremely high so that the potential thereof will not change due to leakage.

At this point it may be seen' that an error signal other than zero applied at terminal 3i will pass through the electrical circuit equivalent toin turn will energize the servo mechanism and result in rotation of shaft 32 and the arm of potentiometer Ra of an amount determined by the magnitude of the error signal at terminal 3|.

Returning now to the original problem of a radar system providing automatic tracking with a spinning antenna, the movement of shaft 32 is utilized in addition to zeroing the potential between terminals I02 and I13, to determine the operation of the circuit producing the tracking gate 26 in Fig. 2. a The jitter of the error signal applied at 3| is, in accordance with the basic principles of this invention, smoothed so that the rotation of shaft 32 corresponds only with the true azimuth movement of the target. In this manner, the rectangular gate 26 is positioned such that its center remains on the true azimuth position of the target. a

This manual movement of the tracking gate is known as slewing.

Summarizing the basis of operation of the circuit disclosed in Fig. 3 comprises the passing of an input signal through a system which effectively removes signal jitter. This is accomplished by an electrical circuit having the characteristics of inertia and damping. When used in connection with radar equipment, a target may be located and tracked accurately and automatically on the information supplied once during each antenna scan.

It is of course possible to utilize the system hereinabove disclosed for various other data smoothing applications, the sole requirement being the application of a pulsed error signal. Thus since various modifications and extensions of the basic principle hereinabove disclosed may become evident to those skilled in the art, it is preferred that this invention be limited not by these specific disclosures but by the spirit and scope of the appended claims.

What is claimed is:

1. Apparatus for providing an output indicative of the trend of an input electrical error signal, said output being substantially independent of error signal jitter, said apparatus comprising a data smoothing circuit, means for applying said error signal to said data smoothing circuit, a position servo operative from said data smoothing circuit and providing said output, said data smoothing circuit being characterized by inertia and damping.

2.-Apparatus for providing an output indicative of the trend of an input electrical error signal and substantially independent of discontinuous error signal jitter, comprising, in combination, a

data smoothing circuit and a position servo mechanism, said position servo providing said output and being operative from said data smoothing circuit, means for obtaining the integral of said error signal, means in said data smoothing circuit including a resistance-capacitance network actuated by the sum of said error signal and said integral of said error signal, said data smoothing circuit being characterized by inertia and damping.

3. Apparatus for providing an output indicative of the trend of an input electrical error sig- In radar systems generally the tracking cir-' cults are inoperativeuntil a target is located;

nal and substantially independent of discontinuous error signal jitter, comprising means for obtaining the integral of said error signal, means for obtaining a combined signal proportional to the sum of said error signal and said integral of said error signal, means for applying said combined signal to a resistance-capacitance network including a plurality of series resistance elements and shunt capaitance elements, said resistance-capacitance network providing the eflects of inertia and damping, a potentiometer, one of said capacitance elements being connected to the movable tap of said potentiometer, said movable tap being operative from a position servoproviding said output, said position servo being energized through said resistance-capacitance network.

4. Apparatus as in claim 3 wherein said means for obtaining the integral of said error signal includes electron tube switching means actuated by said error signal, said error signal comprising a plurality of electrical pulses, and means for charging and discharging said capacitance elements. FRANCIS P. COFFIN.

FRANK E. BOTHWELL. PRESCOTT D. CROUT. 

